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Study on the Mechanical Properties and Microstructure of B4C and B4C/Al Composite Including CeO2

Author: NingFuHu
Tutor: WuWenYuan
School: Northeastern University
Course: Non-ferrous metallurgy
Keywords: boron carbon pressless infiltration hot-pressed sintering mechanical properties toughening mechanism microstructure
CLC: TB333
Type: Master's thesis
Year: 2008
Downloads: 58
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Abstract


B4C materials have been used widely in the fields of electronics、aerospaces、military and other high-tech industries because of its advanced properties such as low density,high chemistry stability and excellent wearable ability,but B4C materials have one fetal defect in the performance.Single B4C is poor in the property of fracture toughness,therefore, 2-3MPa·m1/2 is the best evaluation,which limits the application of B4C materials greatly.In order to solve the problem,the mechanical properties and microstructure of hot-pressed sintered B4C and B4C/Al composite including CeO2 have been studied in this paper.In order to improve the fracture toughness of B4C materials, B4C/Al/CeO2 composite has been fabricated by infiltratingAl in the experiment. B4C/Al/CeO2 composite possesses the excellent properties of metal Al and B4C ceramic, making up the defects of brittleness of B4C material.Besides, B4C/Al/CeO2 composite makes up the shortness of the metal Al, upgrading its hardness.By means of XRD and TG-DTA, the chemical reactions between B4C and Al with the adding amount of CeO2 have been studied in the different temperatures.The results indicate that Al3BC and AIB2 are the resultants in the low temperature, which starts from 625℃to 690℃.The phase of AlB12C2 is the resultant in the temperature range between 1320℃and 1350℃, simultaneously the phase of Al3BC and AlB2 are also the resultants. Moreover, the addition of Rare earth oxide may lead to improve bending strength and fracture toughness through nsitu formation of Rare earth boride.Mechanical properties are tested by the methods of micro-indentation, three-point bending and single edge notched beam.XRD, SEM and TEM technologies are used to determine the microstructure, fracture surface and the phase. The results show that the fracture toughness of B4C/Al/CeO2 composite increase by 75.27% compared with that of single B4C.The microstructure of B4C/Al/CeO2 composite fabricated by pressless infiltration is well-proportioned and compact.The fracture surface of B4C/Al/CeO2 composite shows that the fracture form of the composite material is primarily intergranular fracture.Meanwhile the transgranular fracture along the ceramic skeleton is also existed.Metal Al has the role of improving the toughness. As for B4C/Al/CeO2 composite, improving toughness by metal is the most important way.During the process of the infiltration,Al3BC is formed from the chemical reaction of metal Al and B4C.Al3BC is filled in the gaps of B4C grains,increasing the bonding strength of the interface.The microstructure defects were also detected through TEM.The high-density twin structure observed in line shape by means of TEM was formed along (111) in the sintering process.In order to improve the fracture toughness of B4C material, CeO2 was added to fabricate B4C-CeO2 ceramic material in the method of hot-pressed sintering, which makes no loss to the hardness of B4C material. The SEM results indicate that the microstructure of B4C changes evidently when the adding amount of CeO2 is increased. The most obvious change is the conspicuous fall in the ratio of pore. There is no evidence that the grains grow at a larger size. The density of B4C-CeO2 ceramic material increases constantly as the increase in the quantity of CeO2. The ratios of the sintered density are all above 96%, compared with the theoretical density, among which the highest ratio reaches 96.9%. The mechanical property tests indicate that when the adding amount CeO2 is 2%, the fracture toughness of CeO2-B4C reaches 5.30MPa·m1/2,which is higher than the pure B4C by 44%,besides, its hardness reaches 37.59GPa. By means of XRD experiment, CeB6 is tested in the sintering process of B4C and CeO2 under high temperature. The thermodynamics calculation indicates that CeB6 is formed from the chemical reaction among B4C, CeO2 and C at the temperature of 1417℃.The chemical reaction equation is 3B4C+2CeO2+C=2CeB6+4CO. Rare earth boride through nsitu formation plays a major role to improve the fracture toughness of B4C-CeO2 ceramic composite. Crack deflection and crack bypass caused by the residual stress is the major toughening mechanism. The residual stress arises from the mismatch in the thermal expansion coefficient of the matrix B4C and the second phase particles CeB6.

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